# How to simulate surface tension?

I am trying to create a water drop simulation for measuring hydrophobicity of surface. I don't know how to simulate the contact angle which is related by younges equation to the surface tensions for each of the solid-liquid-gas interfaces. I have come across some models for molecular dynamics, which for chemistry novice like me (I'm a EE) are bit advanced. I am looking for simpler method in which I could simulate the interfacial tensions by some sort of derived mathematical model which would be simple enough to act as an objective function to an evolutionary optimizer.

• I'm not sure I understand. You know about Young's relation between the surface tensions. Do you want something that predicts the surface energies, or something that solves for the contact angle and the shape of the drop? – Geoff Hutchison May 27 '15 at 17:29
• well I need the contact angle so I guess the latter would be what i'm looking for. However based on my understanding of the problem I will have materials which will create three interfaces (solid-liquid, solid-gas, liquid-gas) which I need to generate for my simulation. I am at a loss of how to generate these inter-facial tensions for calculating the contact angle. I realize in some situations the values for these tensions are from lookup tables, however I am not sure whether it would be wise to assume static tension values in a dynamic system (water drops on surface). – RaaziR May 28 '15 at 3:08
• Unless there's something to perturb the system (e.g., rolling the drop down a surface at an angle) the drop will quickly come into equilibrium. If you don't have the surface tensions for a particular material, you'll need some sort of particle simulation. – Geoff Hutchison May 28 '15 at 3:12
• well one of the objectives of the simulation is to generate data to feed to an evolutionary optimizer for a design program. The two main objective functions I have seemingly isolated for are 1.) Contact angle using the cassie-baxter model, and two 2.) the wetting angle (as you mentioned above). Below I have provided the a sample simulator for the hydrophillic case, and the paper which provides the mathematical model for the objective functions. (Hydrophillic) Simulator| Paper – RaaziR May 28 '15 at 3:27

If I understand your question correctly, you want to relate a droplet's geometry to it's environment. The curvature of a liquid droplet is described by the Laplace equation of capillarity, also known as the Young-Laplace equation. This is valid for both a sessile (sitting on a surface) and a pendant (hanging) droplet. I am assuming a static experiment in which the droplet is axisymmetric, with $z$ defined as the axis of symmetry. Such methods of solving for interfacial tension and contact angles are called Axisymmetric Drop Shape Analysis (ADSA) methods. ADSA methods are based on fitting the shape of an experimental droplet to the Laplace equation. This differential equation has been numerically solved in 1883, but thanks to computers and camera's this method is easier and more accurately to apply.

The Laplace equation is given by:

$$\gamma\left(\frac{1}{R_1}+\frac{1}{R_2}\right)=\Delta P\label{eq:laplace}\tag{1}$$

in which $\gamma$ is the surface tension and $\Delta P$ is the pressure difference over the interface. After some work, this may be written as a system of differential equations. $R_1$ and $R_2$ are the principal radii of curvature. $R_1$ is called the meridional curvature, which turns in the plane of the screen you are reading from. Perpendicular to this is the azimuthal curvature $R_2$.

The meridional curvature is defined as:

$$\frac{1}{R_1}=\frac{\partial \phi}{\partial s}$$ The azimuthal curvature is defined as: $$\frac{1}{R_2}=\frac{\sin{\phi}}{x}$$

`In the absence of external forces, other than gravity, the pressure difference is a linear function of the elevation' (Del Rio and Neumann, 1996):

$$\Delta P= \Delta P_0+\left(\Delta\rho\right)gz\label{eq:rio}\tag{2}$$ $\Delta P_0$ is the pressure at the origin, at which the droplet is not perturbed and $R_1=R_2=R_0$. Using \eqref{eq:laplace} this leads to the following expression: $$\Delta P_0 =\gamma\left(\frac{2}{R_0}\right)\label{eq:p0}$$ Combining \eqref{eq:laplace} and \eqref{eq:rio} and filling in the expression for $\Delta P_0$ and the curvatures leads to:

\begin{align} \gamma\left(\frac{\partial \phi}{\partial s}+\frac{\sin{\phi}}{x}\right)&=\frac{2\gamma}{R_0}+\left(\Delta\rho\right)gz\\ \frac{\partial\phi}{\partial s}&=\frac{2}{R_0}+\frac{\left(\Delta\rho\right)gz}{\gamma}-\frac{\sin{\phi}}{x} \end{align} Now the coordinates are transformed to dimensionless coordinates: \begin{align} \bar{x}=\frac{x}{R_0}, \bar{z}=\frac{z}{R_0}, \bar{s}=\frac{s}{R_0} \end{align}

This changes the differential: \begin{align} \frac{\partial\phi}{\partial s}=\frac{\partial\phi}{\partial\left(R_0\bar{s}\right)}=\frac{\partial\phi}{R_0\partial\bar s} \end{align} Multiplying both sides with $R_0$ gives: \begin{align} \frac{\partial\phi}{\bar s}&=2+\frac{\left(\Delta\rho\right)gR_0^2}{\gamma}\bar z-\frac{\sin\phi}{\bar x}\label{eq:phi} \end{align}

Furthermore the following geometric interpretation leads to the remaining differential equations. The arc length $s$ follows the curvature of the interface of the drop. $\phi$ is the tangential contact angle, so a change in $x$ over a change in $s$ is equal to the cosine of $\phi$:

\begin{align} \frac{\partial x}{\partial s} &= \frac{\partial \bar{x}}{\partial \bar{s}} = \cos(\phi)\label{eq:cos} \end{align} Similarly, a change in $z$ over a change in $s$ is equal to the sine of $\phi$: \begin{align} \frac{\partial z}{\partial s} &= \frac{\partial \bar{z}}{\partial \bar{s}} =\sin(\phi)\label{eq:sin} \end{align} The same holds for the dimensionless coordinates, resulting in the given system of differential equations.

Solving the curvature of the droplet is an initial value problem, with the initial values being $x_0$, $z_0$ and $\phi_0$. You can feed this to your numerical differential equation solver, e.g. Matlab's ODE45 function. With this method you will be able to solve the contact angle and surface tension from a given curvature, or draw a curvature for a given contact angle and surface tension.